Monday, May 26, 2008

Defining Deviancy Down at the Intersection of Death, Part One

Imagine if we measured the American economy by estimating Gross Domestic Product once every four years. Forget about measuring other things like employment, unemployment and inflation. And forget about taking monthly or quarterly measurements.

“That’s ridiculous!” you would yell. “How would we understand the different trends in the components of the economy? How would we track the ups and downs? How would we get a complete picture of what’s going on?”

And you’d be right. But guess what: that is how we measure traffic in Montgomery County.

Montgomery County’s Planning Department measures traffic by calculating Critical Lane Volume (CLV) at each of 422 intersections in the county. CLV is the maximum hourly sum of conflicting auto movements, both through traffic and turns, proceeding through an intersection. (You can see how the measurement is constructed on page 6-26 of the 2006 Edition of the Mass Highway Manual.) CLV is dependent on volume. An intersection without cars would have a CLV of zero. As traffic picks up, CLV rises. But in a perfectly gridlocked intersection, no cars would be able to move and CLV would go back down to zero.

The Planning Department measures CLVs at each intersection by sending out traffic surveyors to count cars in both the morning and evening rush. But since Planning has limited resources, the surveyors can only appear at each of the intersections every four years or so. What if the weather is bad? What if there’s an accident nearby? Too bad, the survey results are in. Whatever happened on that one day is assumed to be the case on every day for at least the next four years.

Some of these measurement days are a bit unusual. Of the 422 reported CLVs in the 2008 Highway Mobility Report, 22 were taken in December, 23 were taken in January, 30 were taken in February and 3 were taken in August. Are these representative months of the year for driving conditions? Also, 29 of the CLVs were taken in 2003, 14 were taken in 2002 and 6 were taken in 2001. Can these measurements really be compared to estimates made in 2006 and 2007?

Moreover, the Planning Department assumes that a high CLV means high congestion. In fact, a high CLV means an intersection is carrying a lot of traffic. Remember – it rises with volume. An extremely congested intersection is one where it takes a long time to get through. In that case, the CLV may actually be quite low. How about measuring average delay times or average speeds? The Planning Department reports average speeds in travel runs taken on MD-355, Georgia Avenue, US-29, Norbeck Road, Connecticut Avenue, Clopper Road and Great Seneca Highway in 2005 and 2007, but those runs were one-day spot-checks covering only a fraction of the county’s intersections. Nevertheless, these travel runs provided valuable data and we will cover them in more detail later this week.

So why should you care whether Planning measures traffic badly? For one thing, an intersection’s CLV plays a role in determining whether developers building nearby will have to pay for traffic mitigation measures. If your CLV was measured on a day in which traffic was abnormally low, it might fall below the allowable traffic standard in that policy area. That would make it easier for a developer to escape responsibility for mitigating the effects of any new traffic generated by additional construction.

In Part Two, we’ll look at how the county’s faulty traffic measures have played out at the intersection of Georgia Avenue and Forest Glen Road, also known as the Intersection of Death.